Generally, a video recorder records a luminance signal using a low-carrier FM recording method and records a chrominance signal using a low-band-conversion direct-recording method, as discussed in greater detail below.
Video signals are first separated into luminance signals and chrominance signals. The separated luminance signal is converted into an FM-modulated wave with a low carrier frequency. Here, selection of the frequency of the carrier wave corresponding to the white peak of the white level of a video signal and to the sync tip of a synchronizing signal is important in relation to a signal-to-noise ratio (S/N). The difference between the carrier's frequencies bracketed by the white peak of the white level and the sync tip of the synchronization signal represents a frequency deviation which should be maximized in order to get the most favorable S/N ratio, taking other conditions into account. This is because the frequency deviation is proportional to the amplitude of the output signal after demodulation. In the eight millimeter system, the white peak of the white level is 5.4 Mhz and the sync tip of the synchronization signal is 4.2 Mhz; the frequency deviation is 1.2 Mhz, accordingly. In the VHS system, the white peak of the white level is 4.4 Mhz and the sync tip of the synchronization signal is 3.4 Mhz, while in the beta system, the white peak of the white level is 4.8 Mhz and the sync tip of the synchronization signal is 3.6 Mhz.
The FM modulation characteristics in the video recorder are such that, even if the transmission zone of a magnetic tape and a video head system is narrow, the complete recording can be accomplished therein, and such that, even if the contact state of the magnetic tape and the video head is somewhat poor, the recording quality will still not be unacceptably degraded.
On the other hand, a chrominance signal whose frequency corresponds to 3.58 Mhz+500 Khz in an NTSC system is converted into the signal whose frequency is 743 Khz (629 Khz in the VHS system and 688 Khz in the beta system), which is lower than that of a luminance signal. The chrominance signal is recorded directly using AM modulation., Since the signal is converted into a lower band with respect to the luminance signal, the signal is called a low-band converting chrominance signal. Also, the chrominance subcarrier converted into a low band is called as a low-band converting chrominance subcarder.
The chrominance signal must be forcibly to be converted into a signal having a low frequency so that the recording zone can be used effectively. Also, since the chrominance signal itself is a phase-modulated wave, it is affected by the jitter due to the running imbalance between the magnetic tape and the video head system. Therefore, if the chrominance signal is converted into a signal having a low frequency, the phase-shifting depending on the same amount of the jitter is reduced so that the chrominance change can be prevented.
The low-band converting chrominance signal is incorporated with a low-carrier FM signal of the luminance signals, and the incorporated signal is supplied to the video head system and recorded on the magnetic tape. The low-carrier FM signal performs the alternating-current bias with respect to the low-band converting chrominance signal, so that no-bias recording can be achieved during video signal recording. When the recorded signal is reproduced, the signal is converted by the reverse of the process used in recording the signal, thus, restoring the original signal.
In the conventional device for forming a low-band converting chrominance signal, an oscillating signal having a frequency Fscu (a low-band converting chrominance subcarrier whose frequency is 47.25 Fh in the NTSC system and 46 7/8 Fh in the PAL system, where Fh is the frequency of an horizontal-synchronizing signal) output from the automatic frequency controller (AFC) by using a voltage controlled oscillator (VCO) and an oscillating signal having a frequency Fs (a chrominance subcarrier whose frequency is 455/2 Fh in the NTSC system and 284 -1/4+1/625 Fh in the PAL system) output from a local oscillator (VXO usually employed because of its good stability) are incorporated together at a subconverter which produces an incorporated signal of Fs+Fscu. Thereafter, the incorporated signal of Fs+Fscu is further incorporated in the main convertor with the Y/C-divided chrominance signal, that is, the chrominance signal modulated by Fs, which therefore produces a low-band converting chrominance signal.
It is also necessary for the conventional low-band converting chrominance signal generating device to have a VCO, which is used in the AFC circuit and the local oscillator.
Moreover, since the chrominance subcarrier and the low-band converting chrominance subcarrier in the NTSC and PAL systems do not have an integer multiplier for each other, there is a problem in that the NTSC/PAL video recorder requires separate VCO and VXO devices for each system.